In seawater thermal desalination such as multi-stage flash (MSF), multi-effect (ME) and the like, only about one gallon of distillate may be actually recovered from ten gallons of a total seawater feed stream. The primarily reason for at least this wasteful usage and pumping of seawater is scale forming compounds. The scale is essentially a mixture of magnesium hydroxide, calcium carbonate, and the hydrates of calcium sulfate (gypsum, anhydrite, and hemihydrate). Magnesium hydroxide and calcium carbonate are alkaline compounds, which are formed as a result of the release of carbon dioxide from seawater. The scale formation of the hydrates of calcium sulfate is controlled by their limited aqueous solubilities, wherein the solubilities of anhydrite and hemihydrate plummet with increasing temperatures. The theme of operating thermal desalination methods is essentially based on pre-heating and de-aerating a water source to partially control alkaline scale, dosing a scale inhibitor, and restricting the top brine temperature (e.g., 65-110° C.) and brine concentration factor (e.g., 1.2-1.6) to mainly avoid conditions under which the solubility limits calcium sulfate hydrates are exceeded.
The point is clear; the dilemma in the desalination field, as disclosed in my patent application Ser. No. 16/501,510 and summarized in FIG. 1, is fully pronounced in disadvantageously operating multiple independent MSF trains in a desalination plant (system) as well as multiple independent MED trains in a desalination plant. The dilemma may also extend to disadvantageously impractical combining of MED-MED concept in an independent train, or combining MSF-MED concepts in an independent train.
As used herein, the convenient practice throughout the entire history of the desalination field is that a desalination plant comprises a plurality of independent trains, wherein each train is operated independently by having its own steam supply, a seawater feed stream, a distillate stream, a reject brine stream, a reject cooling seawater stream, a heat rejection section or a heat rejection condenser, and a re-circulated (recycle) brine stream within each independent train[e.g., when applicable as in a recycle brine MSF (RB-MSF) train in a desalination plant]. For example, if a conventional desalination plant comprised eight RB-MSF trains, then each of the trains in the plant would be operated independently.
The dilemma as summarized in FIG. 1, wherein its ascendancy rests squarely on alkaline and sulfate scale issues, is that by ignoring these scale issues, the solution of one problem has, in turn, created several new ones. By preconceiving the illusion of the “impossible” (scale issues are not solvable), the desalination field has been dramatically limiting the “possible” (scale issues are inevitable, wherein they are tolerable within a pre-set limit, thereby solvable only within desalination methods themselves), all solutions begin and end in a locked in circle within this perceived “possible”; thereby failing to see the distorted perception of the “impossible”. When the preconceived “impossible” and perceived “possible” are bordered, however improbable, the perceived “possible” becomes the specialized thinking, whereby it becomes more difficult to deviate from it, nor to generate creative ideas within it, nor even obvious ones. However, scale issues can neither be solved by desalination methods alone, nor desalination methods can be a fortiori pretend to scale issues, nor the desalination field can hold indefinitely the same operating theme as contrived in the 1950s.